fix: add better error handling for uninitialized billing plugin

- Fix TypeError when accessing providerConfig on undefined billing plugin
- Add proper type safety: useBillingPlugin now returns BillingPlugin | undefined
- Add clear error message when plugin hasn't been initialized
- Update README quickstart with concise provider response examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -100,18 +100,21 @@ export default buildConfig({
 const payment = await payload.create({
   collection: 'payments',
   data: {
-    provider: 'stripe',
+    provider: 'stripe',  // or 'mollie' or 'test'
     amount: 5000,        // $50.00 in cents
     currency: 'USD',
     description: 'Product purchase',
     status: 'pending',
   }
 })
-
-// Payment is automatically initialized with Stripe
-console.log(payment.providerId)  // Stripe PaymentIntent ID
 ```
 
+**What you get back:**
+
+- **Stripe**: `providerId` = PaymentIntent ID, `providerData.raw.client_secret` for Stripe.js
+- **Mollie**: `providerId` = Transaction ID, `providerData.raw._links.checkout.href` for redirect URL
+- **Test**: `providerId` = Test payment ID, `providerData.raw.paymentUrl` for interactive test UI
+
 ## Payment Providers
 
 ### Stripe